Charlie Stross pointed out that, traditionally, 75% of readers don't pay into an author's income stream. Trying to remove that 75% from the readership . . .

No one is trying to restrict 75% (or whatever the figure really is) who legally borrow books, I presume mostly from public libraries. What some are trying to do is to tell the 25% percent, that they can, without guilt, grab the book illegally and keep it permanently.